1. Field of the Invention
The present invention relates to a process for forming a deposited film and a process for producing a semiconductor device. More particularly, the present invention relates to a process for forming a tungsten (W) deposited film preferably applicable in wiring a semiconductor integrated circuit device or the like, and a process for producing a semiconductor device using a method of selectively depositing a tungsten film.
2. Related Background Art
Aluminum (Al) has been hitherto mainly used in electrodes or wiring, in electronic devices or integrated circuits which are comprised of semiconductor. Here, Al has a number of advantages such that it is inexpensive, has a high electrical conductivity, and is capable of giving such a dense oxide film on a surface that its inside can be chemically protected and stabilized or a good adhesion to Si can be achieved.
Incidentally, particularly in recent years, it has become required for the wiring to be made finer and to be formed in more multiple layers because of an increase in the degree of integration of integrated circuits such as LSI, etc. Hence, demands for conventional Al wiring has become far severer than ever. As the size is made finer with an increase in the degree of integration, the surface of an LSI or the like is made greatly uneven as a result of oxidation, diffusion, deposition of thin films, etching, etc. For example, electrodes or wiring metals must be deposited without disconnection on the surface having differences in level or must be deposited in via holes having a minute diameter and a large depth. In the case of 4 Mbit or 16 Mbit DRAM (dynamic RAM), etc., the aspect ratio (i.e., via hole depth/via hole diameter) of a via hole in which metals such as Al, etc. must be deposited is 1.0 or more, and the diameter itself of a via hole is 1 .mu.m or less. Hence, a technique is needed which enables deposition of Al in a via hole having a large aspect ratio.
In particular, in order to achieve a sure connection to a device located beneath an insulating film made of SiO.sub.2 or the like, the Al must be deposited in the manner that via holes only of a device can be filled up rather than a film is formed therein.
Then, because of a low melting point of Al which is as low as 660.degree. C. or a weakness in its electromigration, studies have been made on tungsten (W) as a wiring material that substitutes Al.
As a method of forming deposited film of tungsten, a CVD process is proposed in which WF.sub.6 is used as a starting material gas and W is deposited on an Si substrate by reduction reaction with Si which is a constituent atom of the substrate.
For example, U.S. Pat. No. 4,349,408 discloses a process comprising depositing on a silicon substrate a polycrystalline silicon layer doped with oxygen, subsequently depositing thereon a polycrystalline silicon layer doped with phosphorus, thereafter etching the polycrystalline silicon layer, and depositing a tungsten film on the silicon substrate uncovered by the etching.
European Patent Publication No. 216157(A2) discloses a metal depositing process comprising the steps of (1) placing in a chamber a silicon substrate having a silicon dioxide layer, provided with a hole, (2) feeding a gaseous compound of a metal into the chamber to substitute the silicon of the substrate with the metal so that the metal is deposited to the hole and (3) feeding hydrogen gas together with a gaseous metal so that a metal is further deposited on both the surfaces of the metal thus deposited and the silicon dioxide film; wherein an etching gas is fed into the chamber in the course of the step (3) and the etching gas is activated to carry out etching in such a manner that the silicon dioxide layer can be kept in a state in which no metal is present on its surface and also a metal can be further deposited on the metal deposited layer. As an example, this publication discloses the deposition of tungsten (W).
In the processes disclosed in these publications, however, WF.sub.6 is used as a starting material, and the tungsten film is basically deposited as a result of the reaction between WF.sub.6 and Si. Hence, making uniform the deposition of W on Si may result in a great lowering of the rate of deposition, and on the other hand making sure the rate of deposition may bring about its deposition also on SiO.sub.2. Thus, it has been often difficult to stably maintain the selectivity. Further it is difficult to avoid that the Si substrate is corroded because of the materials used or the etching gas. Moreover, in some instances, the rate of deposition is from 30 to 50 .ANG./min at best and the film thickness has a limit of from about 2,000 .ANG. to about 3,000 .ANG. for a stable thickness. There is also an difficulty about the problem of contamination of an interface between the Si substrate and the tungsten film because of the presence of fluorine. There is also plenty of room for improvement in respect of the uniformity of a film.
On the other hand, from the viewpoints of the formation of wiring and the formation of electrodes, a poor surface roughness of a tungsten thin film obtained by a conventional film forming process has brought about inconveniences of the alignment step for a mask and a substrate and the etching step, in a patterning step for the wiring.
More specifically, a tungsten film formed by conventional sputtering and a tungsten film formed by CVD have surfaces with irregularities ranging from several hundred to several thousand .ANG. to have a poor surface morphology. This has often brought about the following disadvantages:
1) Alignment signals are irregularly reflected on the surface, so that the noise level becomes too high to distinguish inherent alignment signals. PA0 2) In order to cover large surface irregularities, a resist film must be made to have a large thickness. This is contradictory to the demands for fineness. PA0 3) The poorness in surface morphology brings about local halation ascribable to the internal reflection in a resist, causing the resist to remain. PA0 4) The poorness in surface morphology makes side walls rugged in accordance with the irregularities in the course of wiring etching step.
Nevertheless, because of the melting point of tungsten which is as high as 3,410.degree. C., there is the advantage that employment thereof as a wiring material makes it possible to carry out a high-temperature treatment in a posterior step in a process of manufacturing a semiconductor device. It has been impossible to ignore this advantage.
As discussed above, in the technical field of semiconductors that have been recently sought to be more highly integrated, there is plenty of room for improvement in order to inexpensively provide a semiconductor device having a high-integration and a high-performance.